1. Field of the Invention
The present invention relates to a lumber drying method and an impregnation processing method to have the lumber absorb various agents such as preservative, mothproofingg agent, preservative/mothproofing agent, anti-ant agent, anti-fungus agent, dimensional stabilizer and resin.
2. Related Art
As shown in FIG. 3, lumbers are conventionally dried, for example, by a drying apparatus 1 consisting of a drying chamber 2 to accommodate lumbers 6, a heating boiler 3 to send heated air into the drying chamber 2, heat stirrers 4 installed in the drying chamber 2 and an exhaust pipe 5 to discharge the water vapor generated from the lumbers 6.
The lumbers 6 are, for example, placed on a carriage 7 and accommodated in the drying chamber 2. These lumbers 6 are cedars having an end face of about 105 mm.times.105 mm and a length of 3 to 4 m. The lumbers 6 are piled up on the carriage 7 with inserting a 1 to 2 cm thick spacer 8 between each pair of lumbers. Further, the initial moisture content of the lumber 6 is 50 to 70%.
The drying apparatus 1 maintains the temperature in the drying chamber 2 at 60 to 70.degree. C. with the heated air sent from the heating boiler 3 to dry the lumbers 6. Drying by this apparatus is continued for 15 to 20 days. The final moisture content is 15 to 20%.
According to the conventional lumber drying method as shown in FIG. 3, however, the heated air is discharged to outside via the exhaust 5 together with the water vapor from the lumbers 6, which makes the energy cost expensive.
Another drawback is that variation of the water vapor in the drying chamber 2 prevents uniform drying of the lumbers 6.
Further, the heated air drying may cause crack in lumbers 6 when the temperature is raised.
In addition, long drying time (15 to 20 days) means that the apparatus can be used only once or twice a month.
The lumber is inferior to other materials in its dimensional stability, uniformity, preservability, moth-proof, flame retardancy and strength. To improve such drawbacks of lumber and to add functions, the lumber is made composite with chemicals by impregnation. For example, large wooden products such as telephone poles and railroad ties are impregnated for preservation and moth-proof processing.
On the other hand, under the situation with shortage of lumber resources and lower prices in international competition, improvement of unused wood species and low quality wood, development of high durability technology and wooden product manufacturing by such methods are eagerly awaited.
In particular, it is expected that woods are improved or provided with new functions for higher additional values in the field of building materials, furniture members and art objects.
As a conventional method to improve drawbacks of or add functions to lumbers, the impregnation method to have the lumber absorb the agent has been known.
Impregnation methods include vacuum/pressure injection, compression and pressurizing methods. Vacuum/pressure injection methods are, for example, disclosed in Chapter II "Research on Function Enhancement by High Impregnation of Lumbers" contained in researches about processing of wooden members for higher functions issued by the Small and Medium Enterprise Agency in October 1990, the article titled "Pressurized Injection Conditions and Their Characteristics in Impregnation of Lumbers" in Vol. 28 of the Research Report from Shimane Prefectural Industrial Engineering Center (1991), and "Lumber Storage (1)--Focusing on Processing Technology--" in "Lumber Industry" Vol. 49, No. 7, 1994.
The vacuum/pressure impregnation method repeats pressurizing and vacuuming to impregnate the lumber with the processing agent.
The above article "Pressurized Injection Conditions and Their Characteristics in Impregnation of Lumbers" has made impregnation of lumbers with the injection conditions changed as follows: 1) Temperature, pressurizing force and pressurizing time of the injection system; 2) Combination of processes such as repetition of vacuuming and pressurizing; and 3) Pressure atmosphere before injection. It is reported that the following results are obtained about these factors.
1) Time and pressure have direct correlations with the injection amount. Temperature affects little in practice. PA1 2) Combination method of vacuuming and pressurizing did not distinctively affect the injection amount and impregnation cross section. PA1 3) The lower the pressure atmosphere before injection is, the more the injection amount becomes, and the impregnation is substantially affected by the injection time. But, the higher, the pressure atmosphere before injection, the deeper the impregnation becomes for the injection amount.
As a compression method, the method disclosed in "Liquid injection into hardly permeable lumbers by compression method" in Nara Prefectural Experimental Forestry Station Report No. 21 (1991), for example, is known. According to this method, the lumber is pressed and crushed in radial direction before pressurized injection for improvement of liquid permeability. In this compression method, it is found that compression in air-dried state before pressurized injection is effective for improvement of permeability at least for small test pieces.
Further, as a pressurizing method, the method disclosed in "Lecture: Mothproofing of Lumbers (13), Mothproofing, Pressurizing Method" in Vol. 33-5 of "Lumber Industry", for example, is known.
According to the pressurizing method, after putting the lumbers in an injection case and sealing it, chemicals required for preservation, mothproof and dyeing are injected under the pressure of 10 to 15 kgf/cm.sup.2 in one to ten odd hours.
The pressurizing method is widely used as a method to have the lumber absorb the largest quantity of chemicals. In particular, railroad ties, telephone poles and base lumbers to be used outdoors for a long time are subjected to pressurized impregnation of preservation agent for a longer service life.
The absorption amount in this method is much larger than that in application, spraying and immersing. Since sufficient absorption and infiltration length can be achieved, this is the most effective processing method.
However, the conventional lumber impregnation methods as described above have drawbacks of necessity of quite costly processing equipment and low production efficiency.
In addition, there is a concern that cells might be destroyed during processing.
Besides, due to difference in permeability of the processing agent depending on species, individual lumbers and sections, it is difficult to uniformly impregnate processing agents deep into the lumbers by the conventional methods.